案例要求忘記了（大概是……）：

學生（學號，姓名，專業，所選課程{<3}）
老師（工號，姓名，所教課程{<3}）
課程（課程號，課程名，學分，教師，已選課學生{<30}）

選課系統程式碼如下：

//teacher

public class Teacher {
    private int id;
    private String teacherName;
    private Course[] courses;
    //建構函式
    public Teacher() {
        super();
        courses= new
 
 Course[3];
    }
    public Teacher(int id,String teacherName){
        this.id=id;
        this.teacherName=teacherName;
        courses = new Course[3];
    }
    //修改或是新增屬性
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getTeacherName
 
() {
        return teacherName;
    }
    public void setTeacherName(String teacherName) {
        this.teacherName = teacherName;
    }


}

     /**
     * 課程
     */

public class Course {
    private String courseName;
    private int courseId;
    private Teacher teacher;
    private float credit;
    private
 
 Student[] students;
    //建構函式
    public Course(int courseId,String courseName,float credit,Teacher teacher) {
        super();
        this.courseId=courseId;
        this.courseName=courseName;
        this.credit=credit;
        this.setTeacher(teacher);
        students = new Student[30];
    }
    public Course(int courseId,String courseName,float credit) {
        super();
        this.courseId=courseId;
        this.courseName=courseName;
        this.credit=credit;
        students = new Student[30];
    }

    public Course(int courseId,String courseName) {
        super();
        this.courseId=courseId;
        this.courseName=courseName;
        students = new Student[30];
    }

    public Course() {//預設形式，要有以防萬一
        super();
        students = new Student[30];
    }

    //修改或獲取屬性值id，name，credit，
    public void setId(int id){
        this.courseId=id;
    }
    public int getId(){
        return this.courseId;
    }
    public void setName(String name){
        this.courseName=name;
    }
    public String getName(){
        return this.courseName;
    }
    public void setCredit(float credit ){
        this.credit=credit;
    }
    public float getCredit(){
        return this.credit;
    }
    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    //課加入學生
    public boolean addStudent(Student stu){
        boolean flag = false;//標誌值：是否加入成功
        //如果學生沒有選過這門課，同時課的學生還沒滿則執行
        if(!isSelectedStudent(stu)&&isNullStudent(stu)){
            for(int i=0;i<students.length;i++){
                if(students[i]==null){
                    students[i]=stu;
                    flag=true;
                    break;
                }
            }
        }
        return flag;
    }
    //課移除學生
    public boolean removeStudent(Student stu){
        boolean flag=false;
        if(isSelectedStudent(stu)){//選過這門課
            for(int i=0;i<students.length;i++){
                if(students[i]==stu){
                    students[i]=null;
                    flag=true;
                    break;
                }
            }
        }
        return flag;
    }
    //顯示選擇課程的學生：
    public void displayStudent(){
        System.out.println("選擇的課程："+this.courseName+"的學生有:");
        for(Student s:students){
            if(s!=null){
                System.out.print(s.getStuName()+" ");
            }
        }
        System.out.println();
    }
    //子方法1：學生是否選過這門課
    public boolean isSelectedStudent(Student stu){
        boolean flag=false;
        for(Student s:students){//只能用於檢查，不能修改
            if(s==stu){
                flag=true;
                break;
            }
        }
        return flag;
    }
    //子方法2：學科學生未達到限定人數嗎
    public boolean isNullStudent(Student stu){
        boolean flag=false;
        for(Student s:students){
            if(s==null){//還有空位
                flag=true;
                break;
            }
        }
        return flag;
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub

    }


}

/**
 * 學生程式碼
 * @author Floris0811
 */
public class Student {
    private String stuName;
    private int stuId;
    private String major;
    private Course[] courses;
    //建構函式
    public Student() {//不要忘
        super();
        courses = new Course[3];
    }
    public Student(int stuId,String stuName) {
        super();
        this.stuId=stuId;
        this.stuName=stuName;
        courses = new Course[3];
    }
    public Student(int stuId,String stuName,String major) {
        super();
        this.stuId=stuId;
        this.stuName=stuName;
        this.major = major;
        courses = new Course[3];
    }
    //修改獲取屬性name,id,major
    public String getStuName() {
        return stuName;
    }

    public void setStuName(String stuName) {
        this.stuName = stuName;
    }
    public int getStuId() {
        return stuId;
    }
    public void setStuId(int stuId) {
        this.stuId = stuId;
    }
    public String getMajor() {
        return major;
    }
    public void setMajor(String major) {
        this.major = major;
    }
    //學生選課；
    public boolean addCourse(Course course){
        boolean flag=false;
        if(!isSelectedCourse(course)&&isNullCourse(course)){
            for(int i=0;i<this.courses.length;i++){
                if(courses[i]==null){
                    courses[i]=course;
                    course.addStudent(this);//課程也要新增學生
                    flag=true;
                    break;
                }
            }
        }
        return flag;
    }
    //學生移除課程
    public boolean removeCourse(Course course){
        boolean flag=false;
        if(isSelectedCourse(course)){
            for(int i=0;i<this.courses.length;i++){
                if(courses[i]==course){
                    courses[i]=null;
                    course.removeStudent(this);//在課程中移除學生
                    flag=true;
                    break;
                }
            }

        }
        return flag;
    }
    //顯示學生所選的課程
    public void displayCourse(){
        System.out.println("學生"+this.stuName+"所選課程有：");
        for(Course c:courses){
            if(c!=null){
                System.out.print(c.getName()+" ");
            }
        }
        System.out.println();
    }

    //子方法1：課是否被選過
    public boolean isSelectedCourse(Course course){
        boolean flag=false;
        for(Course c:courses){
            if(c==course){
                flag=true;
                break;
            }
        }
        return flag;
    }
    //子方法2：學生是否還有選修課位置
    public boolean isNullCourse(Course course){
        boolean flag=false;
        for(Course c:courses){
            if(c==null){
                flag=true;
                break;
            }
        }
        return flag;
    }



}

package test;

public class ChooseCourseByStu {

    /**
     * 選課管理系統
     */
    public static void main(String[] args) {
        Student stu0 = new Student(1001,"Lily");
        Student stu1 = new Student(1002,"Eilly");
        Student stu2 = new Student(1003,"Floris");
        Student stu3 = new Student(1004,"HaHa");
        Course cour0 = new Course(001,"高數");
        Course cour1 = new Course(002,"線代");
        Course cour2 = new Course(003,"概率論");
        stu0.addCourse(cour0);
        stu0.addCourse(cour2);
        stu0.addCourse(cour1);
        stu1.addCourse(cour2);
        stu1.addCourse(cour0);
        stu2.addCourse(cour1);
        stu3.addCourse(cour0);
        stu3.addCourse(cour1);
        stu1.removeCourse(cour2);
        stu0.displayCourse();
        cour0.removeStudent(stu1);
        cour1.displayStudent();
    }

}

That’s it! Over~